Zoom lens system and imaging apparatus

ABSTRACT

A zoom lens system includes: a primary image forming lens group that forms light from an object side into an intermediate image and a relay lens group that forms light from the intermediate image into a final image. The primary image forming lens group includes, in order from the object side, a first fixed lens group G1 with negative refractive power, a stop St, and a second fixed lens group G2 with positive refractive power, and the relay lens group includes, in order from the object side, a third fixed lens group G3 with negative refractive power, a first moving lens group G4 with positive refractive power, and a second moving lens group G5 with positive refractive power.

BACKGROUND

The present invention relates to a zoom lens system and an imagingapparatus that uses such zoom lens system.

Japanese Laid-Open Patent Publication No. 2003-232993 (hereinafter“Document 1”) describes the provision of a refractive optical systemthat forms an intermediate image and is capable of favorably forming theintermediate image in spite of using few lenses. Document 1 describes arefractive optical system including an image forming lens group thatforms an intermediate image of a object, a field lens group disposednear the formation position of the intermediate image, and a relay lensgroup that forms the intermediate image into another image, where thefocal distance f1 of the image forming lens group and the focal distancef of the refractive optical system satisfy the condition 1<|f1/f|<3.

SUMMARY

Among wide-angle zoom lens systems whose full angle of view at thewide-angle end exceeds 110°, there is demand for a compact zoom lenssystem that obtains sharp images.

One aspect of the present invention is a zoom lens system consisting: afirst optical system that forms light from an object side into anintermediate image; and a second optical system that forms light fromthe intermediate image into a final image, wherein the first opticalsystem consists of a fixed lens group that does not move during zooming,the second optical system includes a variator lens group that movesduring zooming, and the fixed lens group includes a first negativemeniscus lens that is disposed closest to the object side and whoseconvex surface is oriented toward the object side.

In this zoom lens system, the first optical system is a lens group ofwide-angle in which a first meniscus lens is disposed closest to theobject side with the convex surface oriented toward the object side.This lens group guides the light flux that has passed the first meniscuslens across the opposite side to the optical axis and forms an invertedimage as an intermediate image, with the final image being formed by thesecond optical system. In this zoom lens system, during zooming, thevariator lens group included in the second optical system moves but thewide-angle and fixed lens group of the first optical system does notmove. Since the first optical system that forms the intermediate imageis fixed, it is possible to also fix the formation position (imagingposition) of the intermediate image. This means there is no fluctuationin the imaging position of the intermediate image that accompanieszooming, and at any position during zooming from the wide-angle end tothe telephoto end, the intermediate image should not be formed on a lenssurface or inside a lens. This means that it is possible to suppressscratches and the like or foreign matter such as dust on lens surfacesfrom appearing in the final image.

In addition, since the position of the intermediate image dose not moveby fixing the first optical system, it is possible to design the lenssystem so that the back focus of the first optical system is shorter. Itbecomes easier to make the lens system more wide angle and to make theoverall length of the first optical system shorter, which makes iteasier to reduce the overall length of the zoom lens system. Inaddition, since the first optical system is fixed during zooming, it ispossible to concentrate the driving mechanism for zooming in theperiphery of the second optical system and the zooming mechanism issimplified. Accordingly, it is possible to provide a compact, wide-anglezoom lens system that is capable of forming a sharp final image.

It is desirable for the fixed lens group to include: a first fixed lensgroup that has negative refractive power and includes a first negativemeniscus lens; a second fixed lens group that has positive refractivepower and is disposed on the final image side of the first fixed lensgroup; and a stop disposed between the first fixed lens group and thesecond fixed lens group. By such configuration, it becomes easy toposition the entrance pupil of the zoom lens system on the object sideof the stop and close to the first fixed lens group. It makes the zoomlens system more wide angle even if the lens aperture of the firstnegative meniscus lens is not increased.

It is also desirable for the second fixed lens group to include a firstcemented lens disposed closest to the object side and for the firstcemented lens to include a biconvex positive lens and a biconcavenegative lens disposed in order from the object side. By disposing thefirst cemented lens on the final image side to the entrance pupil, it ispossible to effectively correct chromatic aberration. It is alsodesirable for the first cemented lens to include a biconvex positivelens disposed on the final image side of the negative lens.

It is desirable for the second optical system to include a third fixedlens group with negative refractive power that is disposed on the objectside of the variator lens group and does not move during zooming. Byfixing the first optical system, it is possible to fix the position ofthe intermediate image, and by also disposing the third fix lens groupbetween the intermediate image and the variator lens group, it ispossible to interpose the intermediate image between the fixed lensgroup and the third fixed lens group. Accordingly, the plane of theintermediate image dose not overlaps with on a lens surface or inside alens during zooming.

It is desirable for the distance on the optical axis between theintermediate image and a lens surface that is closest to the object sideout of the third fixed lens group to be shorter than the distance on theoptical axis between the intermediate image and a lens surface that isclosest to the final image side of the fixed lens group. In this zoomlens system, by fixing the respective lens groups on both the objectside and the final image side of the intermediate image, it is possibleto maintain a relationship whereby the distance between the intermediateimage and the closest lens surface to the object side of the third fixedlens group is shorter than the distance between the intermediate imageand the closest lens surface to the final image side of the fixed lensgroup. According to this relationship, since the front focus of thethird fixed lens group becomes shorter than the back focus of the fixedlens group, it becomes easier to reduce the lens aperture of the thirdfixed lens group.

It is desirable for the third fixed lens group to include a firstpositive meniscus lens that is disposed closest to the final image sideand whose convex surface is oriented toward the final image side, andfor a refractive index n1 of the first negative meniscus lens and arefractive index n3 of the first positive meniscus lens to satisfyConditions (1.1) to (1.3) below.n1≧1.65  (1.1)n3≧1.90  (1.2)0.82≦n1/n3<1.00  (1.3)

In this zoom lens system, by satisfying the Conditions (1.1) to (1.3),it is possible to cancel out the strong negative distortion produced bythe first negative meniscus lens that has a high refractive index tomake the lens system more wide angle with the strong positive distortionin the inverse direction produced by the first positive meniscus lensthat also has a high refractive index. Accordingly, with this zoom lenssystem, it is easy to form a final image where distortion has beenfavorably corrected.

Typically, this zoom lens system can be designed with the negativedistortion DistLf and the positive distortion DistLb satisfy Conditions(1.4) and (1.5) below.DistLf≧−10  (1.4)0.98≦|DistLf|/|DistLb|≦1.02  (1.5)

It is desirable for the third fixed lens group to include a secondcemented lens that is disposed closest to the final image side, for thesecond cemented lens to be composed of a second negative meniscus lenswhose convex surface is oriented toward the final image side and a firstpositive meniscus lens disposed in order from the object side, and forthe Abbe number v32 of the second negative meniscus lens and the Abbenumber v33 of the first positive meniscus lens to satisfy the Conditions(A) and (B) below.45.0≦v32≦55.0  (A)15.0≦v33≦25.0  (B)

In this zoom lens system, Condition (A) is set so that the secondnegative meniscus lens is provided with weak dispersive power (lowdispersion) and Condition (B) is set so that the first positive meniscuslens is provided with strong dispersive power (high dispersion). Thismeans that by combining the second negative meniscus lens that has lowdispersion and the first positive meniscus lens that has highdispersion, it is possible to favorably correct chromatic aberrationfrom the visible region to the near-infrared region.

It is also desirable for the variator lens group to include: a firstmoving lens group with positive refractive power that moves from thefinal image side toward the object side when zooming from the wide-angleend toward the telephoto end; and a second moving lens group withpositive refractive power that is disposed on the final image side ofthe first moving lens group and moves from the final image side towardthe object side when zooming from the wide-angle end toward thetelephoto end, for the first moving lens group to be composed of a firstpositive lens disposed closest to the final image side, for the secondmoving lens group to be composed of a second positive lens, a firstnegative lens, and a third positive lens disposed in order from theobject side, and for the partial dispersion ratio Pgf41 of the firstpositive lens, the partial dispersion ratio Pgf51 of the second positivelens, the partial dispersion ratio Pgf52 of the first negative lens, andthe partial dispersion ratio Pgf53 of the third positive lens to satisfyConditions (2.1) to (2.4) below.0.53≦Pgf41≦0.55  (2.1)0.53≦Pgf53≦0.55  (2.2)0.55≦Pgf51≦0.58  (2.3)0.58≦Pgf52≦0.66  (2.4)

Conditions (2.1) and (2.2) show that the partial dispersion ratio Pgf41of the positive lens closest to the object side out of the first movinglens group and the partial dispersion ratio Pgf53 of the third positivelens closest to the final image side of the second moving lens group areset low with a substantially equal balance. In addition, Condition (2.3)shows that the partial dispersion ratio Pgf51 of the second positivelens is set slightly higher than Conditions (2.1) and (2.2), andCondition (2.4) shows that the partial dispersion ratio Pgf52 of thefirst negative lens is set the highest out of the variator lens group.By satisfying the Conditions (2.1) to (2.4) given above, it is easy tosuppress fluctuations in chromatic aberration that accompany zooming,which makes it easy to form a final image in which various aberrationsare favorably corrected. In addition, by cementing together the firstpositive lens and a lens with positive refractive power that satisfiesCondition (2.1), it is possible to more favorably correct chromaticaberration in the zoom lens system.

A further aspect of the present invention is an imaging apparatusincluding: the zoom lens system described above; and an imaging devicedisposed at a position where the final image of the zoom lens system isformed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an arrangement of a zoom lens system according to a firstembodiment of the present invention and an imaging apparatus that usessuch zoom lens system, with FIG. 1A including the lens arrangement atthe wide-angle end and FIG. 1B including the lens arrangement at thetelephoto end;

FIG. 2 shows lens data of the zoom lens system according to the firstembodiment;

FIG. 3 is a series of tables showing various numeric values of the zoomlens system according to the first embodiment, with FIG. 3A showingfundamental data, FIG. 3B showing zoom data, and FIG. 3C showingaspherical surface data;

FIG. 4 shows aberration graphs of the zoom lens system according to thefirst embodiment, with FIG. 4A showing aberration graphs at the wideangle end and FIG. 4B showing aberration graphs at the telephoto end;

FIG. 5 shows aberration graphs of a first optical system of the zoomlens system according to the first embodiment;

FIG. 6 shows an arrangement of a zoom lens system according to a secondembodiment of the present invention and an imaging apparatus that usessuch zoom lens system, with FIG. 6(a) including the lens arrangement atthe wide-angle end and FIG. 6(b) including the lens arrangement at thetelephoto end;

FIG. 7 is a diagram showing lens data of the zoom lens system accordingto the second embodiment;

FIG. 8 is a series of tables showing various numeric values of the zoomlens system according to the second embodiment, with FIG. 8A showingfundamental data, FIG. 8B showing zoom data, and FIG. 8C showingaspherical surface data;

FIG. 9 shows aberration graphs of the zoom lens system according to thesecond embodiment, with FIG. 9A showing aberration graphs at the wideangle end and FIG. 9B showing aberration graphs at the telephoto end;and

FIG. 10 shows aberration graphs of a first optical system of the zoomlens system according to the second embodiment.

DETAILED DESCRIPTION

FIG. 1 shows the overall arrangement of a zoom lens system 10 accordingto a first embodiment of the present invention and an imaging apparatus1 that uses such zoom lens system 10. FIG. 1A includes the lensarrangement at the wide-angle end (WIDE) and FIG. 1B includes the lensarrangement at the telephoto end (TELE). The imaging apparatus (camera)1 includes the zoom lens system 10 and an imaging device (imagingelement, image sensor) 50, such as a CCD or a CMOS, that is disposed ata position where a final image 52 of the zoom lens system 10 is formedand converts the final image 52 to an electrical signal (image data).

The zoom lens system 10 is composed, in order from a object side(subject side) 10 a, a first optical system (primary image forming lensgroup) 11 that forms light from the object (subject) into anintermediate image 51 and a second optical system (secondary imageforming lens group, relay lens group) 12 that forms light from theintermediate image 51 into the final image 52. The primary image forminglens group 11 is a fixed wide-angle lens group (fixed lens group) 11 athat does not move when the zoom lens system 10 carries out zooming. Thefixed wide-angle lens group 11 a according to the present embodiment iscomposed, in order from the object side 10 a, of a first fixed lensgroup G1 with negative refractive power, a stop (aperture stop St), anda second fixed lens group G2 with positive refractive power.

The relay lens group 12 is composed, in order from the object side 10 a,of a third fixed lens group G3 with negative refractive power that doesnot move during zooming and a variator lens group 12 a that moves duringzooming. The variator (variable magnification) lens group 12 a iscomposed of a first moving lens group G4 with positive refractive powerthat moves during zooming and a second moving lens group G5 withpositive refractive power that moves during zooming. The imaging device50 is disposed on a final image side (or “image side”) 10 b of the relaylens group 12 with a cover glass CG in between. The imaging device 50 iscapable of providing image data to a host apparatus such as a personalcomputer and/or transfers the image data to an external informationprocessing apparatus via a computer network or the like.

The zoom lens system 10 is an image reforming lens system (or “two-stageoptical system”) composed of the primary image forming lens group 11 andthe relay lens group 12, and is composed of a total of thirteen lensesL11, L12, L21 to L25, L31, L32, L41, and L51 to L53 that are made ofglass.

The zoom lens system 10 forms the intermediate image 51 between theprimary image forming lens group 11 and the relay lens group 12 andforms the final image 52 on the final image side 10 b of the relay lensgroup 12. In the primary image forming lens group of this zoom lenssystem 10, the first fixed lens group G1 gathers light flux that isincident from a region (an upper half region, first region, for example)100 a relative to a first plane 100 x that includes the optical axis 100into a direction along the optical axis 100 and the second fixed lensgroup G2 converges the light flux to converge so as to form the invertedintermediate image (inverted image) 51 on the opposite side to the firstregion 100 a in another region (lower half region, second region, forexample) 100 b relative to the first plane 100 x that includes theoptical axis 100. In the relay lens group 12, the third fixed lens groupG3 disperses the light flux emitted from the intermediate image 51 andthe first and second moving lens groups G4 and G5 converge the lightflux while varying the magnification so as to form the final image(upright image) 52, where up-down and left-right of the intermediateimage 51 are inverted, in the first region 100 a.

The first fixed lens group G1 that is closest to the object side 10 a isa lens group which as a whole has negative refractive power and iscomposed of a first negative meniscus lens L11, whose convex surface S1is oriented on the object side 10 a, and a negative meniscus lens L12,which is also convex on the object side 10 a, disposed in that orderfrom the object side 10 a. The first negative meniscus lens L11 is alens with the largest effective diameter (aperture) in the zoom lenssystem 10, and is extremely compact with an effective diameter of 16.50mm. Both surfaces of the negative meniscus lens L12, that is a convexsurface S3 on the object side 10 a and a concave surface S4 on the finalimage side 10 b, are aspherical surfaces.

The second fixed lens group G2 disposed on the final image side 10 b ofthe first fixed lens group G1 with the stop St in between is a lensgroup that as a whole has positive refractive power and is composed of afirst cemented lens (balsam lens) LB1 where three lenses are stucktogether, a biconvex positive lens L24, and a positive meniscus lens L25that is convex on the object side 10 a disposed in that order from theobject side 10 a. The first cemented lens LB1 is composed of a biconvexpositive lens L21, a biconcave negative lens L22, and a biconvexpositive lens L23 disposed in that order from the object side 10 a. Bothsurfaces of the positive lens L24, that is, a convex surface S9 on theobject side 10 a and a convex surface S10 on the final image side 10 bare aspherical surfaces. The distance dmia (d12a) on the optical axis100 between a concave surface S12 on the final image side 10 b of thepositive meniscus lens L25 and the intermediate image 51 is 3.34 mm.

The third fixed lens group G3 disposed on the final image side 10 b ofthe second fixed lens group G2 with the intermediate image 51 in betweenis a lens group that as a whole has negative refractive power and iscomposed of a biconcave negative lens L31 and a first positive meniscuslens L32 whose convex surface S16 is oriented toward the final imageside 10 b disposed in that order from the object side 10 a. The distancedmib (d12b) on the optical axis 100 between the intermediate image 51and the concave surface S13 on the object side 10 a of the negative lensL31 is 2.10 mm.

The first moving lens group G4 disposed on the final image side 10 b ofthe third fixed lens group G3 is a lens group that as a whole haspositive refractive power and is composed of a biconvex positive lens(first positive lens) L41.

The second moving lens group G5 disposed closest to the final image side10 b is a lens group that as a whole has positive refractive power, andis composed of a positive meniscus lens (second positive lens) L51 thatis convex on the object side 10 a, a biconcave negative lens (firstnegative lens) L52, and a biconvex positive lens (third positive lens)L53 disposed in that order from the object side 10 a. Both surfaces ofthe positive lens L53, that is, the convex surface S23 on the objectside 10 a and the convex surface S24 on the final image side 10 b, areaspherical surfaces.

The zoom lens system 10 is a variable magnification (i.e., zoom) lenssystem composed of thirteen lenses L11 to L53 that are grouped into thefive lens groups G1 to G5 that respectively have negative, positive,negative, positive, and positive refractive powers in that order fromthe object side 10 a. With the zoom lens system 10, when zooming fromthe wide-angle end to the telephoto end, the fixed lens groups G1 to G3do not move, that is the image formation magnification for the incidentlight flux is not changed, and by monotonously moving the moving lensgroups G4 and G5 along the optical axis 100 from the final image side 10b toward the object side 10 a, the image formation magnification to theintermediate image 51 is changed. The movement distance of the firstmoving lens group G4 (in the present embodiment, 8.84 mm) is slightlylarger than the movement distance of the second moving lens group G5 (inthe present embodiment, 7.41 mm). Focal adjustments (focusing) may becarried out by any of the first fixed lens group G1, the positivemeniscus lens L25 of the second fixed lens group G2, and the entire zoomlens system 10.

With this zoom lens system 10, the light flux that is received from awide range (a wide angle) via the convex surface S1 of the firstnegative meniscus lens L11 disposed closest to the object side 10 a isguided by the primary image forming lens group 11 across the opticalaxis 100 to the opposite side of the optical axis 100 (the second region100 b) to form an intermediate image (primary image formation) which isthe inverted image 51 and is then guided across the optical axis 100toward the original side (the first region 100 a) of the optical axis100 by the relay lens group 12 and formed into the final image(secondary image formation) which is the upright image 52. That is, byhaving light that is incident from off-axis positions (i.e., off-axisrays) cross the optical axis 100 twice, the final image 52 is formed ina region (the first region 100 a) on the same side of the optical axis100 as the incident light. This means that it is not necessary torefract and form the final image by the wide-angle off-axis light rayswithin the first region 100 a only, which makes the system more wideangle while suppressing the generation of various aberrations.Accordingly, it is also easy to improve design freedom.

With this zoom lens system 10, during zooming, the moving lens groups G4and G5 included in the relay lens group 12 that is closer to the finalimage side 10 b than the intermediate image 51 both move and the primaryimage forming lens group 11 that forms the intermediate image 51 isfixed and does not move. This means that it is possible to fix the imageformation position of the intermediate image 51 so as to not move.Accordingly, there is no fluctuation in the image formation position ofthe intermediate image 51 that accompanies zooming, and at any zoompositions from the wide-angle end to the telephoto end, it is possibleto prevent the plane of the intermediate image from becoming positionedat a lens surface or inside a lens. This means that it is possible tosuppress scratches and the like or foreign matter such as dust on lenssurfaces from appearing in the final image 52. In addition, in the zoomlens system 10, since the second and third fixed lens groups G2 and G3that are closest to the intermediate image 51 are fixed on both sides ofthe intermediate image 51 in the direction of the optical axis 100, itis easy to maintain a seal in the periphery of the intermediate image 51when installing the zoom lens system 10 in the imaging apparatus 1 andpossible to suppress the introduction of foreign matter into theperiphery of the intermediate image 51.

In addition, since it is possible to fix the imaging position of theintermediate image 51 by fixing the primary image forming lens group 11,it is possible to make the back focus of the primary image forming lensgroup 11 extremely short. This means that it is easier to make the lenssystem more wide angle and to make the overall length of the firstoptical system shorter, which makes it easier to reduce the overalllength of the zoom lens system. In addition, since the primary imageforming lens group 11 dose not move during zooming, it is possible toconcentrate the driving mechanism for zooming in the periphery of thefirst and second moving lens groups G4 and G5, it is possible tosimplify the driving mechanism and cams used for zooming. For thisreason, it is easy to reduce the overall length of the zoom lens system10 and to miniaturize the lens size of the object side 10 a.Accordingly, it is possible to provide the zoom lens system 10 that canachieve a wide angle of view with a compact configuration and is capableof forming a bright and sharp final image 52. In the zoom lens system10, the stop St is disposed between the first fixed lens group G1 andthe second fixed lens group G2. The entrance pupil EP of the zoom lenssystem 10 is positioned on the object side 10 a of the stop St, whichmakes it easy to place the entrance pupil close to the first fixed lensgroup G1. In this zoom lens system 10, since the primary image forminglens group 11 internally includes the entrance pupil EP, it is possibleto construct the first fixed lens group G1 that is closer to the objectside 10 a than the entrance pupil EP from only lenses with negativerefractive power. This means that it is possible to effectively increasethe angle of view and to make the lens system more wide angle withoutincreasing the lens apertures of the first negative meniscus lens L11and the negative meniscus lens L12. In addition, by disposing the firstcemented lens LB1 on the final image side 10 b of the entrance pupil EP,it is possible to effectively correct chromatic aberration caused by thefirst negative meniscus lens L11 and the negative meniscus lens L12 thathave different dispersion. Note that the position of the entrance pupilEP is expressed as the distance from the convex surface S1 on the objectside 10 a of the first negative meniscus lens L11 disposed closest tothe object side 10 a and in the present embodiment is 5.46 mm.

In the zoom lens system 10, the positive meniscus lens L25 that has thehighest refractive index (in the present embodiment, 1.96) in the zoomlens system 10 is disposed closest to the final image side 10 b side ofthe primary image forming lens group 11. The high refractive power ofthe positive meniscus lens L25 refracts the principle light rays of theincident light flux (inward) toward the optical axis 100. Accordingly,it is possible to reduce the back focus of the primary image forminglens group 11, that is, the distance d12a between the concave surfaceS12 of the positive meniscus lens L25 and the intermediate image 51.This makes the overall length of the zoom lens system 10 reduce. Inaddition, since it is possible to gather off-axis light rays toward theoptical axis 100 using the positive meniscus lens L25 that has a highrefractive index, it is possible to suppress an increase in the imageheight of the intermediate image 51. It reduces the size of the negativelens L31 that is disposed closed to the object side 10 a of the relaylens group 12 and is the first lens to receive the light flux dispersedfrom the intermediate image 51.

In this zoom lens system 10, since a compact intermediate image 51 isformed by the primary image forming lens group 11, the negative lens L31is placed close to the intermediate image 51. The negative lens L31 isdisposed so that the distance d12b (in the present embodiment, 2.10 mm)between the intermediate image 51 and the concave surface S13 of thenegative lens L31 is shorter than the distance d12a (in the presentembodiment, 3.34 mm) between the concave surface S12 of the positivemeniscus lens L25. Accordingly, it is possible to make the front focusof the third fixed lens group G3 shorter than the back focus of thesecond fixed lens group G2, the lens apertures of the third fixed lensgroup G3 is reduced, and the overall length of the zoom lens system 10becomes shorter.

In this zoom lens system 10, the convex surface S1 of the first negativemeniscus lens L11 of the first fixed lens group G1 that receives theincident light from the object side 10 a and the convex surface S16 ofthe first positive meniscus lens L32 of the third fixed lens group G3that receives the dispersed light from the intermediate image 51 aredisposed as to face in opposite directions, and the zoom lens system 10is designed so that the refractive index n1 of the first negativemeniscus lens L11 and the refractive index n3 of the first positivemeniscus lens L32 satisfy the following conditions (1.1) to (1.3).n1≧1.65  (1.1)n3≧1.90  (1.2)0.82≦n1/n3<1.00  (1.3)

In the zoom lens system 10, by satisfying the Conditions (1.1) to (1.3),by causing the first positive meniscus lens L32 that has a highrefractive index to produce strong pincushion distortion (positivedistortion) in the inverse direction to the strong barrel typedistortion (negative distortion) produced by the first negative meniscuslens L11 that has a high refractive index to make the lens system morewide angle, it is possible to have the positive and negative distortioncancel each other out. Accordingly, it is easy to form the final image52 where distortion has been favorably corrected.

An upper limit in Condition (1.1) should preferably be 2.25, with 2.10more preferable, and 2.00 even more preferable. The lower limit inCondition (1.1) should preferably be 1.75, with 1.85 more preferable. Anupper limit in Condition (1.2) should preferably be 2.25, with 2.10 morepreferable, and 2.00 even more preferable. The lower limit in Condition(1.2) should preferably be 1.92, with 1.94 more preferable.

If the upper limit in Condition (1.1) is exceeded, there is an increasein negative distortion which makes correction difficult, while if thelower limit in Condition (1.1) is exceeded, it becomes difficult to makethe lens system wide angle. If the upper limit in Condition (1.2) isexceeded, there is an increase in positive distortion which makescorrection difficult, while if the lower limit in Condition (1.2) isexceeded, there is a decrease in positive distortion which makescorrection difficult. If the range in Condition (1.3) is exceeded, thebalance between the refractive indices is lost and it is difficult forthe distortion to cancel out.

In the zoom lens system 10, by disposing the negative meniscus lens L12,both of whose surfaces are aspherical, and the positive lens L24, bothof whose surfaces are also aspherical, in the primary image forming lensgroup 11, it is possible to suppress an increase in the negativedistortion DistLf produced by the primary image forming lens group 11.Accordingly, it is easy to cancel out the negative distortion DistLfusing the positive distortion DistLb produced by the relay lens group12. This zoom lens system 10 is designed so that the negative distortionDistLf and the positive distortion DistLb satisfy Conditions (1.4) and(1.5) below.DistLf≧−10  (1.4)0.98≦|DistLf|/|DistLb|≦1.02  (1.5)

The zoom lens system 10 is also designed so that the partial dispersionratio Pgf41 of the positive lens L41, the partial dispersion ratio Pgf51of the positive meniscus lens L51, the partial dispersion ratio Pgf52 ofthe negative lens L52, and the partial dispersion ratio Pgf53 of thepositive lens L53 satisfy Conditions (2.1) to (2.4).0.53≦Pgf41≦0.55  (2.1)0.53≦Pgf53≦0.55  (2.2)0.55≦Pgf51≦0.58  (2.3)0.58≦Pgf52≦0.66  (2.4)

In the zoom lens system 10, the partial dispersion ratio Pgf41 of thepositive lens L41 and the partial dispersion ratio Pgf53 of the positivelens L53 are set low with a substantially equal balance as shown inConditions (2.1) and (2.2), the partial dispersion ratio Pgf51 of thepositive meniscus lens L51 is set slightly higher than Conditions (2.1)and (2.2) as shown in Condition (2.3), and the partial dispersion ratioPgf52 of the negative lens L52 is set the highest out of the variatorlens group 12 a as shown in Condition (2.4). By satisfying theConditions (2.1) to (2.4) given above, it is easy to suppressfluctuations in chromatic aberration that accompany zooming, which makesit easy to form the final image 52 in which various aberrations arefavorably corrected. In addition, by making both surfaces S23 and S24 ofthe positive lens L53, which is the final lens disposed closest to thefinal image side 10 b in the moving lens group G4, aspherical surfaces,it is possible to favorably correct curvature of field and sphericalaberration and also possible to suppress an increase in the number oflenses and suppress an increase the overall optical length.

The upper limits in Condition (2.1) and (2.2) should preferably be 0.54.The upper limit in Condition (2.3) should preferably be 0.57, and thelower limit in Condition (2.3) should preferably be 0.56. The upperlimit in Condition (2.4) should preferably be 0.64, and the lower limitin Condition (2.4) should preferably be 0.60.

FIG. 2 shows lens data of the various lenses of the zoom lens system 10.FIG. 3 shows various numeric values of the zoom lens system 10. In thelens data, “Ri” represents the radius of curvature (mm) of each lens(i.e., each lens surface) disposed in order from the object side 10 a,“di” represents the distance (mm) between the respective lens surfacesdisposed in order from the object side 10 a, “Di” represents theeffective diameter (mm) of each lens surface disposed in order from theobject side 10 a, “nd” represents the refractive index (d line) of eachlens disposed in order from the object side 10 a, and “vd” representsthe Abbe number (d line) of each lens disposed in order from the objectside 10 a. In FIG. 2, “Flat” indicates a flat surface. Also, in FIG. 3A,the entrance pupil position shows the distance from the convex surfaceS1 of the first negative meniscus lens L11 and the exit pupil positionshows the distance from the image plane of the imaging device 50. Thisis also the same in the following embodiments.

As shown in FIG. 3B, the air gap (distance) d16 between the third fixedlens group G3 and the first moving lens group G4, the air gap d18between the first moving lens group G4 and the second moving lens groupG5, and the air gap d24 between the second moving lens group G5 and thecover glass CG all change in the zoom lens system 10.

Also, both surfaces S3 and S4 of the negative meniscus lens L12, bothsurfaces S9 and S10 of the positive lens L24, and both surfaces S23 andS24 of the positive lens L53 are aspherical surfaces. The asphericalsurfaces are expressed by the following expression using thecoefficients K, C1, C2, C3, C4, C5, and C6 shown in FIG. 3C with X asthe coordinate in the optical axis direction, Y as the coordinate in adirection perpendicular to the optical axis, the direction in whichlight propagates as positive, and R as the paraxial radius of curvature.Note that “En” represents “10 to the power n”.X=(1/R)Y2/[1+{1−(1+K)(1/R)2Y2}1/2]+C1Y4+C2Y6+C3Y8+C4Y10+C5Y12+C6Y14

The values in the equations given as Conditions (1.1) to (1.3) and (2.1)to (2.4) described above for the zoom lens system 10 according to thepresent embodiment are as shown below.n1=1.88  Condition (1.1)n3=1.96  Condition (1.2)n1/n3=0.96  Condition (1.3)Pgf41=0.53  Condition (2.1)Pgf53=0.54  Condition (2.2)Pgf51=0.57  Condition (2.3)Pgf52=0.63  Condition (2.4)

Accordingly, the projection lens system 10 in the present embodimentsatisfies Conditions (1.1) to (1.3) and (2.1) to (2.4).

FIG. 4 shows aberration graphs of the zoom lens system 10 with FIG. 4Ashowing aberration graphs at the wide angle end and FIG. 4B showingaberration graphs at the telephoto end. FIG. 5 shows aberration graphsof the primary image forming lens group 11 of the zoom lens system 10.As shown in FIG. 4 and FIG. 5, all of the aberrations are favorablycorrected and it is possible to get sharp images. Note that sphericalaberration is shown for the respective wavelengths of 656 nm (dottedline), 587 nm (solid line), 546 nm (dot-dash line), 486 nm (dot-dot-dashline), and 435 nm (dot-long dash line). In addition, astigmatism isshown separately for tangential rays (T) and sagittal rays (S). Thevertical axis in the aberration graphs shows image height (IMG HT). Thisis also the case for the embodiments described later. Accordingly, thezoom lens system 10 according to the present embodiment is one exampleof a zoom lens system 10 that is wide angle with a full angle of view of126° at the wide angle end, has an f number of 2.0 and is capable offorming sharp images, and is favorably balanced between performance andcost.

FIG. 6 shows the overall configuration of a zoom lens system 20according to a second embodiment of the present embodiment and animaging apparatus 1 that uses such zoom lens system 20, with FIG. 6(a)showing the lens arrangement at the wide-angle end (WIDE) and FIG. 6(b)showing the lens arrangement at the telephoto end (TELE).

The second optical system 20 is also composed of a primary image forminglens group 21 that forms the light from the object side 20 a into theintermediate image 51 and a relay lens group 22 that forms the lightfrom the intermediate image 51 into the final image 52 disposed in thatorder from the object side 20 a. The primary image forming lens group 21according to the present embodiment is also a fixed wide-angle lensgroup 21 a that does not move when the zoom lens system 10 carries outzooming. The fixed wide-angle lens group 21 a is also composed, in orderfrom the object side 20 a, of a first fixed lens group G1 with negativerefractive power that does not move during zooming, a stop St, and asecond fixed lens group G2 with positive refractive power that does notmove during zooming.

The relay lens group 22 in the present embodiment is also composed, inorder from the object side 20 a, of a third fixed lens group G3 withnegative refractive power that does not move during zooming and avariator lens group 22 a that moves during zooming. The variator lensgroup 22 a is composed of a first moving lens group G4 with positiverefractive power that moves during zooming and a second moving lensgroup G5 with positive refractive power that moves during zooming.

As a whole, this zoom lens system 20 is composed of a total of fourteenlenses L11, L12, L21 to L25, L31 to L33, L41, and L51 to L53 that aremade of glass.

The third fixed lens group G3 is composed of a biconcave negative lensL31 and a second cemented lens (balsam lens) LB2 where two lenses arestuck together disposed in that order from the object side 20 a. Thesecond cemented lens LB2 is composed of a second negative meniscus lensL32 whose convex surface S16 is oriented toward the final image side 20b and a first positive meniscus lens L33 whose convex surface S17 isoriented toward the final image side 20 b disposed in that order fromthe object side 20 a. Note that since the compositions of the other lensgroups G1, G2, G4, and G5 and the forms of the lenses included in suchlens groups G1, G2, G4, and G5 are the same as in the first embodiment,the same reference numerals have been assigned and description of theindividual lenses is omitted.

In this zoom lens system 20, the second cemented lens LB2 is disposedcloser to the final image side 20 b than the intermediate image 51 andthe second optical system 20 is designed so that the Abbe number v32 ofthe second negative meniscus lens L32 and the Abbe number v33 of thefirst positive meniscus lens L33 satisfy Conditions (A) and (B) below.45.0≦v32≦55.5  (A)15.0≦v33≦25.0  (B)

In this zoom lens system 20, Condition (A) is set so that the secondnegative meniscus lens L32 is provided with weak dispersive power (lowdispersion) and Condition (B) is set so that the first positive meniscuslens L33 is provided with strong dispersive power (high dispersion). Bycombining the second negative meniscus lens L32 that has low dispersionand the first positive meniscus lens L33 that has high dispersion, it ispossible to favorably correct chromatic aberration from the visibleregion to the near-infrared region. Accordingly, even in cases where theincident light is in a wavelength band that includes near-infrared andthe correction of chromatic aberration by the first cemented lens LB1disposed closer to the object side 20 a than the intermediate image 51is insufficient, it is possible to reinforce the chromatic aberrationcorrection capability using the second cemented lens LB2. If the rangesof Conditions (A) and (B) are exceeded, there is an increase in axialchromatic aberration and aberration correction becomes difficult.

FIG. 7 shows lens data of the various lenses of the zoom lens system 20.FIG. 8 shows various numeric values of the zoom lens system 20. As shownin FIG. 8B, the air gap d17 between the third fixed lens group G3 andthe first moving lens group G4, the air gap d19 between the first movinglens group G4 and the second moving lens group G5, and the air gap d25between the second moving lens group G5 and the cover glass CG allchange in the zoom lens system 20. Also, both surfaces S3 and S4 of thenegative meniscus lens L12, both surfaces S9 and S10 of the positivelens L24, and both surfaces S24 and S25 of the positive lens L53 areaspherical surfaces.

The values in the equations given as Conditions (1.1) to (1.3), (2.1) to(2.4), (A), and (B) described above of the zoom lens system 20 accordingto the present embodiment are as shown below. Note that in the presentembodiment, the refractive index n3 indicates the refractive index ofthe first positive meniscus lens L33.n1=1.88  Condition (1.1)n3=1.95  Condition (1.2)n1/n3=0.96  Condition (1.3)Pgf41=0.53  Condition (2.1)Pgf53=0.53  Condition (2.2)Pgf51=0.55  Condition (2.3)Pgf52=0.64  Condition (2.4)v32=46.6  Condition (A)v33=18.0  Condition (B)

Accordingly, the projection lens system 20 in the present embodimentsatisfies Conditions (1.1) to (1.3), (2.1) to (2.4), (A), and (B).

FIG. 9 shows aberration graphs of the zoom lens system 20 with FIG. 9Ashowing aberration graphs at the wide-angle end and FIG. 9B showingaberration graphs at the telephoto end. FIG. 10 shows aberration graphsof the primary image forming lens group 21 of the zoom lens system 20.As shown in FIG. 9 and FIG. 10, all of the aberrations are favorablycorrected and it is possible to project sharp images. Note thatspherical aberration is shown for the wavelength of 900 nm using abroken line.

Accordingly, the zoom lens system 20 according to the present embodimentis wide angle with a full angle of view of 117° at the wide angle end,has an f number of 1.8 and is capable of forming sharp images from thevisible region to the near-infrared region. This means that it ispossible to provide the zoom lens system 20 that is favorable for ahigh-resolution camera (imaging device) that does not have an autofocusmechanism or the like and is used in combined day and night combinedmonitoring or a variety of applications such as observation, disasterprevention, measurement, and recording.

Note that the present invention is not limited to such embodiments andis defined by the scope of the claims. That is, the primary opticalsystem and/or the secondary optical system of the zoom lens system mayinclude at least one reflective surface (mirror). Also, the invention asdefined by each claim may be regarded as being independent to the otherclaims.

The invention claimed is:
 1. A zoom lens system, consisting: a firstoptical system that forms light from an object side into an intermediateimage; and a second optical system that forms light from theintermediate image into a final image, wherein the first optical systemconsists of a fixed lens group that is fixed throughout an entire zoomrange of the zoom lens system, the second optical system includes avariator lens group that moves during zooming, and the fixed lens groupincludes a first negative meniscus lens that is disposed closest to theobject side and whose convex surface is oriented toward the object side,wherein the second optical system includes a third fixed lens group withnegative refractive power that is disposed on the object side of thevariator lens group and does not move during zooming; wherein the thirdfixed lens group includes a first positive meniscus lens that isdisposed closest to the final image side and whose convex surface isoriented toward the final image side, and a refractive index n1 of thefirst negative meniscus lens and a refractive index n3 of the firstpositive meniscus lens satisfy following conditionsn1≧1.65n3≧1.900.82≦n1/n3<1.00.
 2. The zoom lens system according to claim 1, whereinthe fixed lens group includes: a first fixed lens group that hasnegative refractive power and includes the first negative meniscus lens;a second fixed lens group that has positive refractive power and isdisposed on the final image side of the first fixed lens group; and astop disposed between the first fixed lens group and the second fixedlens group.
 3. The zoom lens system according to claim 2, wherein thesecond fixed lens group includes a first cemented lens disposed closestto the object side, and the first cemented lens includes, in order fromthe object side, a biconvex positive lens and a biconcave negative lens.4. The zoom lens system according to claim 3, wherein the first cementedlens includes a biconvex positive lens disposed on the final image sideof the negative lens.
 5. The zoom lens system according to claim 1,wherein the distance on the optical axis between the intermediate imageand a lens surface that is closest to the object side out of the thirdfixed lens group is shorter than the distance on the optical axisbetween the intermediate image and a lens surface that is closest to thefinal image side of the fixed lens group.
 6. The zoom lens systemaccording to claim 1, wherein the third fixed lens group includes asecond cemented lens that is disposed closest to the final image side,the second cemented lens is composed of a second negative meniscus lenswhose convex surface is oriented toward the final image side and thefirst positive meniscus lens disposed in order from the object side, andthe Abbe number v32 of the second negative meniscus lens and the Abbenumber v33 of the first positive meniscus lens satisfy followingconditions45.0≦v32≦55.015.0≦v33≦25.0.
 7. A zoom lens system, consisting: a first optical systemthat forms light from an object side into an intermediate image; and asecond optical system that forms light from the intermediate image intoa final image, wherein the first optical system consists of a fixed lensgroup that is fixed throughout an entire zoom range of the zoom lenssystem, the second optical system includes a variator lens group thatmoves during zooming, and the fixed lens group includes a first negativemeniscus lens that is disposed closest to the object side and whoseconvex surface is oriented toward the object side, wherein the variatorlens group includes: a first moving lens group with positive refractivepower that moves from the final image side toward the object side whenzooming from the wide-angle end toward the telephoto end; and a secondmoving lens group with positive refractive power that is disposed on thefinal image side of the first moving lens group and moves from the finalimage side toward the object side when zooming from the wide-angle endtoward the telephoto end, wherein the first moving lens group iscomposed of a first positive lens disposed closest to the final imageside, the second moving lens group is composed of a second positivelens, a first negative lens, and a third positive lens disposed in orderfrom the object side, a partial dispersion ratio Pgf41 of the firstpositive lens, a partial dispersion ratio Pgf51 of the second positivelens, a partial dispersion ratio Pgf52 of the first negative lens, and apartial dispersion ratio Pgf53 of the third positive lens satisfyfollowing conditions.0.53≦Pgf41≦0.550.53≦Pgf53≦0.550.55≦Pgf51≦0.580.58≦Pgf52≦0.66.
 8. An imaging apparatus comprising: the zoom lenssystem according to claim 1; and an imaging device disposed at aposition where the final image of the zoom lens system is formed.
 9. Animaging apparatus comprising: the zoom lens system according to claim 7;and an imaging device disposed at a position where the final image ofthe zoom lens system is formed.